The latest in high-tech broadcast radio, Satellite Digital Audio Radio Service or System (SDARS), is capable of providing a new level of service to the subscribing public. SDARS promises to overcome several perceived limitations of prior broadcast forms. All such prior forms are “terrestrial,” meaning that their broadcast signals originate from Earth-bound transmitters. As a result, they have a relatively short range, perhaps a few hundred miles for stations on the AM and FM bands. Therefore, mobile broadcast recipients are often challenged with constant channel surfing as settled-upon stations slowly fade out and new ones slowly come into range. Even within range, radio signals may be attenuated or distorted by natural or man-made obstacles, such as mountains or buildings. Radio signals may even wax or wane in power or fidelity depending upon the time of day or the weather.
Additionally, broadcast radio is largely locally originated. This constrains the potential audience that will listen to a particular station and thus the money advertisers are willing to pay for programming and on-air talent. While the trend is decidedly toward large networks of commonly-owned radio stations with centralized programming and higher-paid talent, time and regulatory change are required to complete the consolidation.
Finally, the Federal Communications Commission (FCC) defined the broadcast radio spectrum decades ago, long before digital transmission and even digital fidelity were realizable. The result is that the bandwidth allocated to a FM radio station is not adequate for hi-fidelity music, and the bandwidth allocated to an AM radio station is barely adequate for voice. This is especially true in a mobile environment.
SDARS promises to change all of this. A user who has an SDARS receiver in his vehicle (or home) can tune into any one of a hundred or more nationwide stations with the promise of near compact disc (CD) quality digital sound. Satellite redundancy and transcontinental coverage substantially provide immunity to service interruption both locally and on long trips.
While SDARS uses satellites for broad-area coverage, SDARS providers typically complement their satellite signals with gap-filling redundant broadcasts using terrestrial stations located in regions having poor or no satellite reception, such as cities with tall buildings, bridges and tunnels. The signals broadcast from the satellite and by the terrestrial stations contain the same audio data, and are typically on adjacent frequencies but use different coding techniques. The terrestrial signals are also typically broadcast at significantly higher signal strength, primarily because terrestrial stations have easy access to electrical power while satellites are limited to the electrical power available from their solar panels.
To promote competition in SDARS, the U.S. Government has divided the 25 MHz S-band allocated to SDARS into two equal 12.5 MHZ subbands and licensed those subbands to two independent service providers: Sirius Satellite Radio of NY, N.Y. and XM Satellite Radio of Washington, D.C. Each service provider operates its own independent transmission system, including its own constellation of satellites and its own network of terrestrial repeaters. The repeaters are located mostly, of course, in urban areas. FIG. 1 shows the relative frequencies and power levels of the signals in the Sirius system. Two geo-synchronous satellites transmit S band (2.3 GHz), time division multiplexed (TDM) signals directly to the end user's receiver. The terrestrial stations broadcast a coded orthogonal frequency division multiplexed (CODFM) signal containing the same audio data. The terrestrial COFDM signals are also broadcast at an S band frequency, lying between the frequencies of the two satellite TDM signals, and at a significantly higher power level.
The terrestrial repeater signals tend to be stronger than the satellite signals and because the Sirius and XM SDARS services occupy proximate subbands, the signals of one provider can interfere with the signals of the other causing degradation of the audio quality. A particular concern arises when a terrestrial repeater of one service introduces noise into the satellite signals of the other service. The noise plays havoc with the way SDARS receivers interpret the signals they are trying to receive.
FIG. 3 is a diagram of a prior art SDARS receiver 100 designed to receive and decode audio channels contained within the SDARS signals. The receiver 100 includes two decoding circuits 111 and 138, the former for decoding TDM signals directly from the satellites and the latter for decoding COFDM terrestrial signals. The combined signals—COFDM, TDM1 and TDM2—are received at a common antenna/low noise amplifier (LNA)/cable unit 102. TDM2 is a delayed version of TDM1. The receiver includes some front end processing before the decoding circuits 111, 138, including RF filter 104, such as a ceramic filter, a variable gain RF amplifier 106, an image rejection filter 108 and an RF mixer 112. Amplifier 106 amplifies the combined signal—COFDM, TDM1 and TDM2—centered at 2326.25 MHz. RF power detector 110 reports the RF power level to the TDM AGC controller 136 and to COFDM AGC controller 158, which will adjust the gain of the RF Amplifier 106 accordingly. RF Mixer 112 down-converts the combined signal to a first IF frequency, such as 315 MHz, which is bandpass filtered by first IF filter 114 and then split into two paths by splitter 116. One output of splitter 116 is applied to the TDM path. It is first applied to the TDM first IF amplifier 118, which is a variable gain amplifier. Following the TDM first IF amplifier 118, the TDM IF mixer 120 downconverts the combined signal to a second IF frequency, such as 75 MHz, which is bandpass filtered by TDM second IF filter 122 and applied to TDM second IF amplifier 124, which is also a variable gain amplifier. The output of the TDM second IF amplifier 124, which contains a downconverted and filtered version of the combined signal, is sampled by the TDM analog-to-digital converter (A/D converter) 126, at a TDM A/D sample rate, such as 60 MHz, with a TDM bit width, such as 10 bits.
The digitized signal from the TDM A/D converter 126 is then split and applied to both the TDM1 digital downconverter (DDC) 128 and the TDM2 digital downconverter (DDC) 130. With appropriate filtering, the TDM1 DDC 128 selects only the TDM1 signal and digitally downconverts it to a baseband signal of TDM1 bandwidth such as 4.5 MHz, and a TDM1 baseband sampling rate such as 30 MHz. With appropriate filtering, the TDM2 DDC 130 selects only the TDM2 signal and digitally downconverts it to a baseband signal of TDM2 baseband bandwidth such as 4.5 MHz, and a TDM2 baseband sampling rate such as 30 MHz. The TDM1 and TDM2 baseband signals are then demodulated with TDM1 Demodulator 132 and TDM2 Demodulator 134, respectively.
In the COFDM path, the other output of splitter 116 is first applied to the COFDM first IF amplifier 142, which is a variable gain amplifier. Following the COFDM first IF amplifier 142, the COFDM IF mixer 144 downconverts the combined signal to a second IF frequency, such as 75 MHz, which is bandpass filtered by COFDM second IF filter 146, which has a bandwidth narrow enough to filter out most of the TDM1 and TDM2 signals. The downconverted COFDM signal is then applied to COFDM second IF amplifier 148, which is also a variable gain amplifier. The output of the COFDM second IF amplifier 148, which contains a downconverted and filtered version of the COFDM signal, is sampled by the COFDM analog-to-digital converter (A/D converter) 150, at a COFDM A/D sample rate, such as 60 MHz, with a COFDM bit width, such as 10 bits.
The digitized signal from the COFDM A/D converter 150 is applied to the COFDM digital downconverter (DDC) 152. With appropriate filtering, the COFDM DDC 152 selects the COFDM signal and digitally downconverts it to a baseband signal of COFDM bandwidth such as 4.1 MHz, and a COFDM baseband sampling rate such as 30 MHz. The COFDM baseband signal is then demodulated with COFDM demodulator 156.
The A/D converters 126 and 150 each have a limited dynamic range. For a 10-bit A/D converter the dynamic range is about 60 dB. The size of the dynamic range plays an important role in digital radio reception. As long as the digitized signal is an accurate representation of the incoming analog signal, digital filtering techniques make it possible to extract very weak signals, such as those received from a satellite, even in the presence of a significant amount of noise. Accurate digitization requires that the incoming signal is amplified sufficiently to fill as much of the A/D converter's dynamic range as possible. It is, however, also very important not to over amplify the incoming signal since, when the A/D is overdriven and overflows, a small signal in a noisy background can be completely lost. This happens because the A/D converter simply truncates any excess signal.
The appropriate gain settings for IF variable gain amplifiers 118 and 124 of the TDM stage 111 that amplify the incoming signal to the optimal level for A/D converter 126 are controlled by TDM Automatic Gain Controller (AGC) 136. TDM AGC 136 controls the amplifiers 118 and 124 in response to the input signal level determined by the RF Power Detector 110 and the demodulated output signal levels from TDM1 Demodulator 132 and TDM2 Demodulator 134, labeled “TDM1 Post-filter” and “TDM2 Post-filter” in FIG. 3. TDM AGC 136 essentially monitors the two demodulated TDM signals and uses the stronger of the two demodulated TDM signals to set the gain of the amplifiers so that the portion of the received signal containing the best TDM signal is amplified appropriately, and a constant output level is obtained. TDM AGC 136 provides a control signal (labeled “TDM IF Gain”) for controlling the gain of amplifiers 118 and 124 to amplify components of TDM 1 and TDM2 according to the algorithm of TDM AGC 136.
IF Variable gain amplifiers 142 and 148 of the COFDM stage 138 are controlled by COFDM AGC 158. Likewise the gain of RF amplifier 106 is controlled by COFDM AGC 158. The control signals “COFDM IF Gain” and “RF Gain” are provided by COFDM AGC 158 in response to the input signal levels from RF Power Detector 110, demodulated signal “COFDM Post-filter” from COFDM Demodulator 156 and digital down converted signal “COFDM Pre_filter” from COFDM DDC 152.
TDM AGC 136 and COFDM AGC 158 are incorporated within a microcontroller that monitors the digitized signal strength levels from the RF and IF elements, as well as the real and imaginary values from the matched filter within the demodulators, to calculate the desired gain control signals to maintain the signal levels in the linear region of the A/D converters 126, 150. The update rate of the IF automatic gain control (i.e. signals TDM IF Gain and COFDM IF Gain) is set at an IF gain update rate, such as 100 Hz. The update rate of the RF automatic gain control (i.e., signal RF gain) is set at an RF gain update rate, such as 50 Hz.
The prior art SDARS receiver 100 utilizes two analog front ends and at least two A/D converters, both of which undesirably consume power and contribute to implementation expense. Further, the receiver 100 tracks the overall TDM signal level instead of individual TDM1 and TDM2 levels separately, which results in sub-optimal performance for TDM reception. The receiver is also inefficient in that it includes separate TDM and COFDM AGC algorithms.
It is desirable to have a receiver with a single path to the analog-to-digital conversion, particularly from a power consumption concern. Practical implementation of a single front-end circuit of the type shown in FIG. 3 is not, however, simple. A major problem in such a circuit is that the amplifier gain settings for the two types of signals may be incompatible with each other. This causes difficulties if the amplifier gain is controlled using a simple, two-state AGC, with one state to optimize the gain for a COFDM signal and one state to optimize the amplifier gain for a TDM signal. In such a system, an amplifier gain that is optimal for the weak TDM signals from the satellite will typically over-amplify the incoming COFDM signal from the terrestrial stations, resulting in the COFDM signal overflowing the A/D converter's dynamic range. This overflow of the A/D converter's dynamic range results in demodulated COFDM audio data of very poor quality, and may even result in not being able to demodulate the COFDM audio at all. This overflow may also “blind” the receiver to the presence of the TDM signals.
Similarly, if the amplifier gain setting is optimal for the A/D converter to digitize the portion of the signal containing the stronger, COFDM signal, the portion of the signal containing the TDM signal will be under-amplified and poorly digitized by the A/D converter. The result is that if the receiver does lock on to a terrestrial COFDM signal, it may stay locked onto the terrestrial signal even if there is a better satellite signal available.
Therefore, improvements are desired in order to realize the power and cost savings attainable with an SDARS receiver using a single analog path and A/D.